Extrudable hot-melt pressure-sensitive adhesives for resealable packaging having improved organoleptic properties

ABSTRACT

Using the said film for resealable cartons.

A subject-matter of the present invention is a novel extrudable hot-meltpressure-sensitive adhesive composition and also a multilayer filmintended for the manufacture of resealable packagings (or cartons),which comprises an adhesive layer composed of the said composition andwhich exhibits improved organoleptic properties.

Extrudable hot-melt pressure-sensitive adhesive compositions intendedfor use for the manufacture of resealable packagings (or cartons) areknown in particular by Applications WO 02/064694, WO 12/045950 and WO12/045951.

Resealable cartons are used in the food processing industry and massmarketing for packaging foodstuffs, in particular fresh products. Suchpackagings are described by U.S. Pat. No. 4,673,601 and PatentApplication EP 1053952.

After the packaging has been opened for the first time and a portion ofthe foodstuff present therein has been consumed, the consumer canmanually reseal the packaging in a substantially hermetic fashion andconsequently provide, if appropriate after placing in a refrigerator,the storage of the remaining portion of the foodstuff. A sequence ofreopenings and resealings is also possible.

These packagings generally comprise a container (or receptacle) and aseal forming a lid, which are hermetically attached to one another bywelding.

The receptacle, which is more or less deep and relatively rigid, iscomposed of a multilayer sheet (also described as complex or compositesheet) having a minimum thickness of 200 μm, generally between 200 and1000 μm. This sheet is thermoformed, so as to exhibit a flat bottom, onwhich the foodstuff rests, and a perimeter in the form of a flat band.This perimeter, generally parallel to the bottom, is bonded by weldingto the flexible and flat seal, which is composed of a multilayer film(also described as complex or composite film), generally with athickness of between 40 and 150 μm, and which is sometimes denoted bythe name of sealing film.

During the opening of the packaging, the sealing film is manuallyseparated from the receptacle at the flat band of the perimeter. Thisoperation results in the appearance of an adhesive layer at this flatband, both on the sealing band and on the receptacle band which werepreviously in contact. These two adhesive layers (continuous ornoncontinuous) referred to as “daughters” result from the rupturing ofan initial or “mother” adhesive layer or, possibly, from its separation(or detachment) from one of the two layers of the multilayer complexfilm which are adjacent to it. The initial adhesive layer is thus one ofthe layers of the said multilayer complex film, which is itself anelement included either in the composite sheet which forms thereceptacle or, optionally, in the sealing film.

The two daughter adhesive layers which are present, after opening thepackaging, on the bands located on the respective perimeter of thereceptacle and of the seal are thus facing one another. Thus, it issufficient to reposition the seal over the receptacle, in accordancewith their position in the packaging before opening, in order to bringthe two bands of daughter adhesive layers back into contact. Simplemanual pressure then makes it possible to obtain the resealing of thepackaging.

The adhesive composition which forms the mother and daughter adhesivelayers is thus necessarily a pressure-sensitive adhesive (PSA).

The extrudable pressure-sensitive adhesive compositions described in theabovementioned patent applications are hot-melt compositions comprisinga tackifying resin and a styrene block copolymer including an elastomerblock. These are substances, solid at ambient temperature, whichcomprise neither water nor solvent. Applied in the molten state, theysolidify when they are cooled, thus forming an adhesive layer whichprovides the bonding between the two thin layers of thermoplasticpolymer material to be assembled, while providing the correspondingpackaging with advantageous opening and resealing properties.

Furthermore, these hot-melt pressure-sensitive adhesive compositions,which are prepared by hot melting the ingredients thereof, additionallyexhibit the advantage of being able to be put into the form of granules(with a size of between 1 and 10 mm) by means of a stage of extrusioncarried out directly under hot conditions after the mixing stage, forexample by means of a twin-screw extruder provided with a tool forcutting the extruded product.

By virtue of the granules thus obtained, the film, for examplethree-layer film, which is composed of the layer of hot-meltpressure-sensitive adhesive composition and of the two thin layers ofthermoplastic polymer material to be assembled can be convenientlymanufactured by coextrusion, by feeding in particular a blown filmdevice with the constituent materials of the three layers in the form ofgranules having the size defined above.

However, in the context of the ongoing improvement in the packagings offoodstuffs provided by the industry to the consumer, it now appearsincreasingly necessary to consider the impact of the said packagings ona possible detrimental change in the organoleptic properties of thepackaged foodstuffs and in particular on a risk of detrimental change intheir taste and/or in their odour (or fragrance), in particular in thecase of hermetic packagings.

As regards the hot-melt pressure-sensitive adhesive compositions forresealable packagings known from the prior art, it is considered thatthis impact can result from the presence in the said compositions ofvery small amounts—ranging up to a maximum limit of 5 ppm—of volatileorganic compounds of low molar mass (less than 1000 Da) which originatefrom the impurities present in the starting materials: for exampleresidues of the monomers or oligomers which are employed in thereactions for the polymerization of the styrene block copolymers ortackifying resins.

The risk thus cannot be ruled out of, in the final packaging, volatileorganic compounds present in the form of traces in the hot-meltpressure-sensitive adhesive composition migrating through the layers ofthermoplastic materials of the complex film to penetrate the foodstuffand to modify the organoleptic properties thereof. This risk can evenless be ruled out if it is considered that some people, highly sensitiveto odour and/or taste sensations, are capable of detecting tracesranging down to 1 ppb of certain substances.

Patent Application WO 14/020243 describes, for this same organolepticproblem, a hot-melt pressure-sensitive adhesive composition which is aconstituent of a layer which bonds two thin layers of thermoplasticmaterial, in a multilayer film, and which necessarily comprises ahydrophobic zeolite.

It is an aim of the present invention to provide another extrudablehot-melt pressure-sensitive adhesive composition than that described bythis last patent application which makes it possible, without, however,requiring the presence of a hydrophobic zeolite, to confer, on themultilayer film of which it constitutes the adhesive layer, a reducedrisk of detrimental change in the organoleptic properties of thepackaged foodstuff and more particularly in its taste.

Another aim of the present invention is to provide an extrudablehot-melt pressure-sensitive adhesive composition exhibiting an improvedhomogeneity in appearance, including in the adhesive layer included inthe multilayer film.

Another aim of the present invention is to provide an extrudablehot-melt pressure-sensitive adhesive composition and also a multilayerfilm which comprises two thin layers of thermoplastic materials bondedto one another by a layer of the said composition and which providesthese two thin layers with opening and resealing properties which aresuitable for the use of the said film for the manufacture of aresealable packaging.

Another aim of the present invention is to manufacture the saidmultilayer film by an industrial process for hot coextrusion of theconstituent materials of each layer, such as blown film coextrusion,comprising the introduction of the said materials in the form ofgranules.

A subject-matter of the present invention is thus, first, a hot-meltpressure-sensitive adhesive composition a having a melt flow index (orMFI) ranging from 0.01 to 200 g/10 minutes and comprising, on the basisof the total weight of the said composition a:

-   -   from 45 to 70% by weight of a composition a1 of styrene block        copolymers comprising at least one elastomer block, the said        composition a1 being composed, on the basis of its total weight:        -   of 30 to 90% by weight of at least one diblock copolymer of            Styrene-Isoprene (SI) type, and        -   of 10 to 70% by weight of at least one            Styrene-Isoprene-Styrene (SIS) linear triblock copolymer;            the total content of styrene units of the said composition            a1 varying from 10 to 40% by weight on the basis of the            total weight of a1; and    -   from 30 to 55% by weight of at least one tackifying resin a2        having a softening temperature of between 5 and 150° C. and        obtained by a process comprising:        -   a first stage of polymerization of a composition chosen            from:            -   a composition (i) essentially composed of unsaturated                hydrocarbons having 9 carbon atoms, or            -   a composition (ii) essentially composed of                dicyclopentadiene and of its derivatives having 10                carbon atoms; then        -   a second stage of hydrogenation of the polymer thus            obtained.

This is because it has been found that the choice of the specifictackifying resin a2 combined with a choice of a styrene block copolymerof SIS/SI type makes it possible, surprisingly, to obtain a multilayerfilm which is suitable for the manufacture of resealable packagings forfoodstuffs and which in addition makes it possible to lower the risk ofdetrimental change in the organoleptic properties of the packagedfoodstuffs. Furthermore, the hot-melt pressure-sensitive adhesivecomposition according to the invention can be extruded, which makes itpossible to present it in the form of granules with a size of between 1and 10 mm, which is particularly advantageous for the manufacture of thecorresponding multilayer film.

The above percentages and also those used generally in the present textto express amounts correspond, unless otherwise indicated, toweight/weight percentages. It is also specified that the percentagesindicated above for the contents, in the composition a, of thecomposition a1 of block copolymers and of the tackifying resin a2 arepercentages relating to the total weight of the said composition a. Itis the same for the percentages indicated for the optional componentsincluded in the composition a which are specified subsequently in thepresent text.

The SI and SIS copolymers included in the composition a according to theinvention have a weight-average molar mass M_(w) of between 60 kDa and400 kDA. Unless otherwise indicated, the weight-average molar massesM_(w) which are given in the present text are expressed in daltons (Da)and are determined by Gel Permeation Chromatography, the column beingcalibrated with polystyrene standards.

According to a preferred alternative form of the hot-meltpressure-sensitive adhesive composition a according to the invention,the content of SI diblocks in the composition a1 of styrene blockcopolymers can vary from 50 to 80% and more preferably still from 55 to80%.

According to yet another preferred alternative form, the total contentof styrene units of the composition a1 of the styrene block copolymerscan vary from 13 to 18%.

The copolymers of SIS and SI type which are capable of being employedfor the preparation of the hot-melt pressure-sensitive adhesivecomposition a are commercially available. The said copolymers arethemselves available in the form of triblock/diblock compositions.Kraton® D1113BT from Kraton and Quintac® 3520 from Zeon Chemicals areexamples of such compositions.

Kraton® D1113BT is a composition for which the total content of styreneunits is 16% and which is composed of 45% of SIS linear triblockcopolymer with an M_(w) of approximately 250 kDa and of 55% of SIdiblock copolymer with an M_(w) of approximately 100 kDa. Quintac® 3520is a composition which is respectively composed of 22% and 78% of SISlinear triblock (M_(w) approximately 300 kDa) and SI diblock (M_(w)approximately 130 kDa) and for which the total content of styrene unitsis 15%.

The hot-melt pressure-sensitive adhesive composition a according to theinvention comprises at least one (or more) tackifying resin a2. Thisresin a2 has a weight-average molar mass M_(w) generally of between 300and 5000 Da. The softening temperature (or point) of the tackifyingresins which can be used in the composition a can vary from 5 to 150° C.

The softening temperature (commonly denoted in the art by the symbol R&B(Ring and Ball) is determined in accordance with the standardized testASTM E 28, the principle of which is as follows. A brass ring with adiameter of approximately 2 cm is filled with the test resin in themolten state. After cooling to ambient temperature, the ring and thesolid resin are placed horizontally in a thermostatically controlledbath of glycerol, the temperature of which can vary by 5° C. per minute.A steel ball with a diameter of approximately 9.5 mm is centred on thedisc of solid resin. The softening temperature is—during the phase ofrise in the temperature of the bath at a rate of 5° C. per minute—thetemperature at which the disc of resin yields by a height of 25.4 mmunder the weight of the ball.

A softening temperature of between 80 and 150° C. is more particularlypreferred.

The process for obtaining the resin a2 is carried out, in a 1st stage,by polymerization of the constituent compounds of the compositions (i)or (ii), which are, for example, obtained from the distillation ofnaphtha. The unsaturated hydrocarbons of the composition (i) may or maynot be aliphatic.

The hydrogenation of the polymer obtained is carried out according toknown methods, resulting in partial or complete hydrogenation of theunsaturated bonds, preferably total hydrogenation.

Many resins corresponding to the definition of the resin a2 arecommercially available. Mention may be made, as examples of such resins,of:

-   -   Regalite™ R1125, available from Eastman, which is a completely        hydrogenated resin having a softening temperature of 125° C. and        a weight-average molar mass M_(w) of 1200 Da and which is        obtained by polymerization of a composition (i) of unsaturated        hydrocarbons having 9 carbon atoms;    -   Escorez™ 5340, available from ExxonMobil, which is a completely        hydrogenated resin having a softening temperature of 140° C. and        a weight-average molar mass M_(w) of 680 Da and which is        obtained by polymerization of a composition (ii);    -   Escorez™ 5400, available from ExxonMobil, which is a partially        hydrogenated resin having a softening temperature of 100° C. and        a weight-average molar mass M_(w) of 570 Da and which is        obtained by polymerization of a composition (ii).

Mention may also be made, as other examples of commercially availableresins a2, of Regalite™ R1090 and R1100 from Eastman (respective R&Bvalues of 90° C. and 100° C.), Arkon™ P90, Arkon™ P100 and Arkon™ P125,available from Arakawa (respective R&B values of 90° C., 100° C. and125° C.), Escorez™ 5300 and Escorez™ 5380 from ExxonMobil (respectiveR&B values of 105° C. and 85° C. and respective M_(w) values of 590 and460 Da) and Sukorez™ SU90, Sukorez™ SU100 and Sukorez™ SU 120 from Kolon(respective R&B values of 90° C., 100° C. and 120° C.).

According to a preferred alternative form, the hot-meltpressure-sensitive adhesive composition a according to the inventioncomprises from 50 to 70% of the composition a1 and from 30 to 50% of thetackifying resin (or resins) a2.

According to another preferred alternative form, the composition a isessentially composed of the composition a1 and of the tackifying resina2 in amounts within the ranges specified above.

The term “essentially composed” is understood to mean that, apart fromthe composition a1 and the tackifying resin (or resins) a2, thecomposition a also includes one or more optional components but in anamount not exceeding 5% and preferably 2%.

Such optional components, the content of which in the composition a canrange from 0.1 to 2%, are, for example, stabilizers (or antioxidants).These compounds are introduced in order to protect the composition fromdecomposition resulting from reaction with oxygen which is capable ofbeing formed by the action of heat, light or residual catalysts oncertain starting materials, such as tackifying resins. These compoundscan include primary antioxidants, which trap free radicals and aregenerally substituted phenols, such as Irganox® 1010 from CIBA. Theprimary antioxidants can be used alone or in combination with otherantioxidants, such as phosphites, for example Irgafos® 168, also fromCIBA, or also with UV stabilizers, such as amines.

Mention may also be made, as example of such optional components, of aplasticizer, the content of which in the composition a does not exceed5%. Use may be made, as plasticizer, of a paraffinic and naphthenic oil(such as Primol® 352 from Esso) optionally comprising aromatic compounds(such as Nyflex 222B).

Finally, other examples of such optional components are inorganic ororganic fillers, pigments or dyes.

The melt flow index (MFI) of the hot-melt pressure-sensitive adhesivecomposition a is measured at 190° C. and for a total weight of 2.16 kg,in accordance with condition d) of Standard ISO 1133. The MFI is theweight of composition (placed beforehand in a vertical cylinder) whichflows in 10 minutes through a die having a fixed diameter, under theeffect of a pressure exerted by a loaded piston having a total weight of2.16 kg. Unless otherwise mentioned, the MFI values indicated in thepresent text were measured under these same conditions.

The hot-melt pressure-sensitive adhesive compositions having an MFIranging from 2 to 70 g/10 minutes are more particularly preferred.

According to an alternative form very particularly preferred for thepurpose of the manufacture of the multilayer film, the composition aaccording to the invention is provided in the form of granules with asize of between 1 and 10 mm, preferably between 2 and 5 mm.

The composition a can be prepared in this form by a process whichcomprises:

-   -   a simple stage of mixing the ingredients under hot conditions,        between 150 and 200° C., preferably at approximately 160° C., by        means of a twin-screw extruder provided with a tool for cutting        the extruded product at the outlet of the die, then    -   a stage of cooling, for example at ambient temperature.

A subject-matter of the present invention is, secondly, a multilayerfilm comprising two thin layers B and C of thermoplastic material bondedtogether by an adhesive layer A, characterized in that the said layer Ahas a thickness of between 7 and 50 μm and is composed of the hot-meltpressure-sensitive adhesive composition a as defined above.

The adhesive layer A preferably provides the bonding between alaminatable thin layer B and a sealable and cleavable thin layer C.

The thickness of the adhesive layer A is preferably between 7 and 35 μmand more preferably still between 7 and 25 μm.

According to another preferred alternative form, the thickness of thelayer A can also be between 10 and 35 μm or better still between 10 and25 μm.

The laminatable layer B can be laminated with other layers for thepreparation of the packaging, for example with a rigid layer for thepreparation of the receptacle.

The sealable and cleavable layer C makes it possible to provide, at theperimeter along which the receptacle is bonded by welding to the seal,the first opening of the packaging, by means of a cleavable embrittledregion. After opening, the embrittled region results in the appearanceof:

-   -   the mother adhesive layer on the sealing band and/or on the        receptacle band which were in contact in the closed packaging,        and/or    -   two daughter adhesive layers which result from the rupturing of        the mother adhesive layer and which are located on the sealing        band and/or the receptacle band.

The material which can be used to form the two layers B and C isgenerally a thermoplastic polymer (identical or different for the twolayers), such as:

-   -   polyethylene (PE),    -   polypropylene (PP),    -   a copolymer based on ethylene and propylene,    -   polyamide (PA),    -   polyethylene terephthalate (PET), or also    -   an ethylene-based copolymer, such as, for example, a copolymer        grafted with maleic anhydride, a copolymer of ethylene and vinyl        acetate (EVA), a copolymer of ethylene and vinyl alcohol (EVOH),        or a copolymer of ethylene and an alkyl acrylate, such as methyl        acrylate (EMA) or butyl acrylate (EBA),    -   polystyrene (PS),    -   polyvinyl chloride (PVC),    -   polyvinylidene fluoride (PVDF),    -   a lactic acid polymer (PLA), or    -   polyhydroxyalkanoate (PHA).

It is preferable to use, to form the two layers B and C, a polyolefinicmaterial, very particularly PE and more preferably still a low densityPE (also denoted by the name of Low Density PolyEthylene or LDPE).

The thickness of the layers B and C is capable of varying within a widerange extending from 5 to 150 μm.

According to a preferred alternative form, the multilayer film accordingto the invention comprises, in addition to the layers A, B and C, twotie layers D and E such that the adhesive layer A is:

-   -   connected to the layer B via the layer D, and    -   connected to the layer C via the layer E.

The tie layers D and E are also denoted by the name of “intermediatelayers” and generally have a thickness of between 1 and 10 μm,preferably between 2 and 8 μm. The use of such tie layers advantageouslymakes it possible to improve the quality of the resealing of themultilayer film and thus of the corresponding resealable packaging, thusproviding the consumer with more effective storage of the remainingportion of the perishable foodstuff present in the packaging, after thelatter has been opened for the first time.

The tie layers D and E are respectively composed of compositions d ande, which are identical or different, which have a melting point fromapproximately 80 to 120° C. and essentially comprise polymers chosenfrom:

-   -   ethylene homopolymers or copolymers,    -   propylene homopolymers or copolymers,    -   copolymers of ethylene with a polar comonomer, and    -   grafted olefin homopolymers or copolymers.

Reference is made, for further information with regard to thesecompositions d and e, to Patent Application US2013/0029553.

Preferably, the compositions d and e each comprise a polyethylene orpolypropylene modified by a cyclic anhydride of an unsaturateddicarboxylic acid having from 4 to 8 carbon atoms.

Maleic anhydride is a very particularly preferred cyclic anhydride.

“Polyethylene modified” by the said anhydride is intended to denoteeither a copolymer of ethylene and the said anhydride or an ethylenehomopolymer or copolymer grafted by the said anhydride.

According to another alternative embodiment, the multilayer filmaccording to the invention comprises, in addition to the three essentiallayers A, B and C and the two optional layers D and E, other thin layersnecessary for the preparation of the packaging, such as, for example:

-   -   a rigid layer necessary for the mechanical strength of the        receptacle, or    -   a printable layer, or    -   a layer having a barrier effect against oxygen, water vapour or        else carbon monoxide.

The materials which can be used to form the said layers can be identicalor different and generally comprise thermoplastic polymers which can bechosen from the polymers mentioned above for the layers B and C.

According to an alternative embodiment, the multilayer film according tothe invention is a film comprising three layers consisting of theadhesive layer A and the two layers B and C, according to the sequenceB/A/C in which the “/” sign means that the faces of the layers concernedare in direct contact.

According to another alternative embodiment, the multilayer filmaccording to the invention is a film comprising five layers consistingof the adhesive layer A, the two intermediate layers D and E and the twoexternal layers B and C, according to the sequence B/D/A/E/C in whichthe “/” sign means that the faces of the layers concerned are in directcontact.

The present invention also relates to a process for the manufacture ofthe multilayer film as defined above, characterized in that it comprisesa stage of coextrusion of the hot-melt pressure-sensitive adhesivecomposition a and of the constituent materials of the layers B and Cand, if appropriate, of the layers D and E.

Preferably, the constituent compositions and materials of the layers A,B, C and, if appropriate, D and E are fed into the coextrusion device inthe form of granules with a size of between 1 and 10 mm, preferablybetween 2 and 5 mm. Thus, the pressure-sensitive adhesive composition aemployed in the multilayer film according to the invention makes itpossible, particularly advantageously, to provide both the propertiesrequired for the said film and the possibility of a presentation of thesaid composition a in the form of the abovementioned granules. The otherlayers possibly included in the multilayer film can be obtained eitherby the incorporation, in the coextrusion device, of the correspondingconstituent materials in the form of granules of the same size (such as,for example, for the tie layers D and E) or by a process of laminatingthe film directly resulting from the coextrusion, for example employinga polyurethane-based adhesive.

The coextrusion device employed in the manufacturing process ispreferably a bubble blowing coextrusion device (also known under thename of “blown film coextrusion device”). In a way known to a personskilled in the art, this process comprises:

-   -   the melting, in separate extruders, of the constituent        compositions and materials of the layers A, B and C and also, if        appropriate, D and E, then    -   the passing of the corresponding streams through an assembly of        annular and concentric dies, so as to form a tubular bubble        comprising several layers, in the order corresponding to that        desired for the final structure, then    -   the radial expansion (relative to the annular die) and the        drawing (in the axial direction) of the bubble, then    -   the cooling of the bubble.

The geometrical characteristics of the dies, as well as the parametersof the process, such as the degree of radial expansion and the drawingrate, are set so as to obtain the thicknesses desired for the variousconstituent layers of the multilayer film. Reference is made inparticular, for a further description of the bubble blowing coextrusionprocess, to Patent Application US2013/0029553.

The present invention also relates to the use of the multilayer film asdescribed above for the manufacture of resealable packagings.

The use for the manufacture of resealable cartons is particularlyadvantageous and, according to a particularly preferred embodiment, forthe manufacture of the sealing film of these cartons.

The following examples are given purely by way of illustration of theinvention and should not under any circumstances be interpreted aslimiting the scope thereof.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forthuncorrected in degrees Celsius and, all parts and percentages are byweight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding application No. FR 14.56561, filedJul. 8, 2014, are incorporated by reference herein.

EXAMPLE A (REFERENCE)

1. Preparation of a hot-melt pressure-sensitive adhesive composition a:

The adhesive composition shown in the following Table is prepared in theform of granules with a size of approximately 4 mm by mixing itsingredients at 160° C. using a twin-screw extruder, extruding through adie, then cutting up the extruded product and cooling to ambienttemperature.

The MFI measured is 57 g/10 minutes.

2. Preparation of a three-layer B/A/C film comprising a layer A of thecomposition of Example A:

This three-layer film is manufactured using a bubble blowing coextrusionpilot-scale device operated continuously, in which three extruders arefed:

-   -   for one, with the composition a of Example A, and    -   for the other two, with LDPE;        the three compositions being in the form of granules with a size        of approximately 4 mm.

The parameters of the process are adjusted so as to manufacture athree-layer film composed:

-   -   as layer A, of a layer with a thickness of 15 μm consisting of        the composition of Example A,    -   as laminatable thin layer B, of a layer with a thickness of 30        μm consisting of LDPE;    -   as sealable and cleavable thin layer C, of a layer with a        thickness of 15 μm also consisting of LDPE.

Mention may be made, among the parameters normally fixed, of a degree ofradial expansion of the bubble equal to 3, a drawing rate of 7 m/minuteand an overall throughput of 11 kg/hour.

The three-layer film thus obtained has a total thickness of 60 μm and alength of 50 m and is packaged in the form of a reel with a machinewidth of 250 mm.

Measurement of the Force of First Opening by T-Peeling at 23° C.:

A sample in the form of a rectangular sheet with A4 format (21×29.7 cm)is cut out from the three-layer film thus obtained.

The external face of the laminatable layer B of this sample is:

-   -   in a first step, subjected to a corona surface treatment (using        a plasma), then    -   in a second step, laminated to a PET film with a thickness of 23        μm using a polyurethane-based solvent-based adhesive and using a        coating device of the Mayer bar type.

The rectangular sheet is then placed under a press for 24 h.

The said rectangular sheet is then folded along a line located in itsmiddle and parallel to the small side of the rectangle, resulting in thesealable and cleavable layer C being brought into contact with itself.

Partial sealing is then carried out using two heating clamping jaws at130° C. applied under a pressure of 6 bar for 1 second, so as to obtainsealed regions of rectangular shape (8 cm in length and 1 cm in width)positioned perpendicularly to the folding line. Each sealed region iscut out in order to obtain a tensile test specimen in which the sealedregion with a length of 8 cm is extended (at that of its ends which isopposite the folding line) by two strips of approximately 2 cm in lengthleft free and unsealed.

These two free strips are attached to two holding devices (known asjaws) respectively connected to a stationary part and a movable part ofa tensile testing device which are located on a vertical axis. Thistensile testing device is a dynamometer.

While a drive mechanism imparts a uniform rate of 300 mm/minute to themovable part, resulting in the peeling of the two sealed layers, theends gradually move along a vertical axis with the formation of an angleof 180°. A force sensor connected to the said movable part measures theforce withstood by the test specimen thus held. The measurement iscarried out in a climate-controlled chamber maintained at a temperatureof 23° C.

The force obtained is shown in the Table.

Measurement of the Force of Second Opening by T-Peeling at 23° C.:

The two parts of the preceding test specimen are, after peeling,repositioned facing one another and brought into contact manually. Theyare then subjected to a pressure exerted by means of a roller with aweight of 2 kg, with which a to-and-fro movement is carried out along adirection parallel to the length of the test specimen.

A tensile test specimen is thus obtained which is identical in shape tothat prepared for the preceding peel test, which is then repeated.

The force obtained is shown in the Table.

Test for evaluating the detrimental change in the taste of butterbrought into contact with the adhesive composition:

This test is carried out on a 125 g block of butter having dimensions of110×65×17 mm.

This block is centred horizontally on a rectangle having dimensions of110×65 mm cut out from the sheet of three-layer film with the A4 formatmentioned above, the block being in contact with the sealable andcleavable layer C of the said film.

The upper face of the block of butter is subsequently covered with asecond rectangle of three-layer film identical to the first, and is alsoin contact with the sealable and cleavable layer C.

The structure thus obtained is overwrapped in tinfoil and then placedfor 10 days in a refrigerator maintained at 10° C.

As control, a block of butter identical to the preceding one, which issimply wrapped in tinfoil, is also stored in the same refrigerator forthe same time.

After 10 days, the two blocks of butter are recovered, are each cut upinto 20 pieces weighing approximately 6 g and are passed on to a panelof ten people for evaluation of the taste. Each member of the panelassigns, after tasting, a grade of 0 to 4 to the pieces resulting fromthe block brought into contact with the three-layer film comprising theadhesive layer A, in comparison with the pieces resulting from thecontrol block, according to the following scale:

-   -   0=no perceptible difference    -   1=barely perceptible difference    -   2=slight difference    -   3=marked difference    -   4=strong difference.

The mean grade obtained is shown in the Table.

EXAMPLES 1 AND 2 (ACCORDING TO THE INVENTION)

Example A is repeated with the compositions a shown in the Table.

The MFI obtained for each composition a and the forces measured for the1st and 2nd opening are shown in the Table, along with the results ofthe tests for evaluating the detrimental change in the taste of thebutter.

It is observed that the force of 1st opening remains substantiallystable with respect to that measured for the reference Example A. On theother hand, the force of 2nd opening is lowered but is assessed ascorresponding to a quality of resealing (following the first opening ofthe packaging) which is still acceptable.

Furthermore, the detrimental change in the taste of the butter resultingfrom the composition a is very significantly reduced in comparison withExample A.

TABLE Content as % weight/weight Example A Ingredient (ref.) Example 1Example 2 Composition a Kraton ® D1113BT 59.7 39.8 39.8 Quintac ® 3520 —19.9 19.9 Escorez ® 1310LC 24.9 — — Regalite ® R1125 — 39.8 — Escorez ®5340 — — 39.8 Dercolyte ® S115 14.9 — — Irganox ® 1010 0.5 0.5 0.5Composition a1 Content of styrene units 16 16 16 (as % weight/weight)Content of diblocks (as % 56 63 63 weight/weight) Composition a MFI (ing/10 min) 57 56 56 Three-layer film Force of 1st opening 3 2.8 3.1 (inN/cm) Force of 2nd opening 0.84 0.5 0.6 (in N/cm) Detrimental change inthe 0.4 0.2 0.1 taste of the butter

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A hot-melt pressure-sensitive adhesive composition a having a meltflow index ranging from 0.01 to 200 g/10 minutes and comprising, on thebasis of the total weight of the said composition a: from 45 to 70% byweight of a composition a1 of styrene block copolymers comprising atleast one elastomer block, the said composition a1 comprising, on thebasis of its total weight: of 30 to 90% by weight of at least onediblock copolymer of Styrene-Isoprene (SI) type, and of 10 to 70% byweight of at least one Styrene-Isoprene-Styrene (SIS) linear triblockcopolymer; the total content of styrene units of the said composition a1varying from 10 to 40% by weight on the basis of the total weight of a1;and from 30 to 55% by weight of at least one tackifying resin a2 havinga softening temperature of between 5 and 150° C. and obtained by aprocess comprising: a first stage of polymerization of a compositioncomprising dicyclopentadiene and its derivatives having 10 carbon atoms;then a second stage of hydrogenation of the polymer thus obtained. 2.The hot-melt pressure-sensitive adhesive composition according to claim1, wherein the content of SI diblocks in the composition a1 varies from50 to 80%.
 3. The hot-melt pressure-sensitive adhesive compositionaccording to claim 1, wherein the softening temperature of thetackifying resin a2 is between 80 and 150° C.
 4. The hot-meltpressure-sensitive adhesive composition according to claim 1, whichconsists of the composition a1 and a tackifying resin a2 and optionallyone or more components in an amount not exceeding 5%.
 5. The hot-meltpressure-sensitive adhesive composition according to claim 1, whose meltflow index ranges from 2 to 70 g/10 minutes.
 6. The hot-meltpressure-sensitive adhesive composition according to claim 1, which isin the form of granules with a size of between 1 and 10 mm.
 7. Amultilayer film comprising two thin layers B and C of thermoplasticmaterial bonded together by an adhesive layer A, wherein the said layerA has a thickness of between 7 and 50 μm and consists of the hot-meltpressure-sensitive adhesive composition a as defined in claim
 1. 8. Themultilayer film according to claim 7, wherein the thickness of theadhesive layer A is between 7 and 35 μm.
 9. The multilayer filmaccording to claim 7, comprising, in addition to the layers A, B and C,two tie layers D and E such that the adhesive layer A is: connected tothe layer B via the layer D, and connected to the layer C via the layerE.
 10. The multilayer film according to claim 9, wherein the tie layersD and E consist of compositions d and e, which are identical ordifferent, which have a melting point from approximately 80 to 120° C.and comprise polymers chosen from: ethylene homopolymers or copolymers,propylene homopolymers or copolymers, copolymers of ethylene with apolar comonomer, and grafted olefin homopolymers or copolymers.
 11. Athree-layer film according to claim 7, which consists of the adhesivelayer A and the two layers B and C, according to the sequence B/A/C inwhich the “/” sign means that the faces of the layers concerned are indirect contact.
 12. A film comprising five layers according to claim 9,which consists of the adhesive layer A, the two intermediate layers Dand E and the two external layers B and C, according to the sequenceB/D/A/E/C in which the “/” sign means that the faces of the layersconcerned are in direct contact.
 13. A process for the manufacture ofthe multilayer film as defined in claim 7, which comprises a stage ofcoextrusion of the hot-melt pressure-sensitive adhesive composition aand of the constituent materials of the layers B and C and, ifappropriate, of the layers D and E.
 14. The manufacturing processaccording to claim 13, wherein the constituent compositions andmaterials of the layers A, B, C and, optionally D and E are fed into thecoextrusion device in the form of granules with a size of between 1 and10 mm.
 15. The process for the manufacture of the multilayer filmaccording to claim 13, wherein the coextrusion device is a bubbleblowing coextrusion device.
 16. A resealable packaging, comprising themultilayer film as defined in claim
 7. 17. The hot-meltpressure-sensitive adhesive composition according to claim 4, whichconsists of the composition a1 and a tackifying resin a2 and one or morecomponents in an amount not exceeding 5%, which are selected from thegroup consisting of stabilizers, antioxidants, plasticizers, inorganicfillers, organic fillers, pigments and dyes.